1. Field of the Invention
The present invention relates to a laser diode driving circuit having a feed-forward type APC (Auto Power Control) circuit for compensating variation of peak power of light output due to variation of the mark rate in an optical transmitter supporting burst mode.
2. Description of the Related Art
In a PON (Passive Optical Network) system (for example, ATM-PON (Asynchronous Transfer Mode-Passive Optical Network)) shown in FIG. 1, the transmission line of a fiber from a exchange is dispersed to plural fibers by a star coupler. In order to perform interactive transmission/reception through a single-core fiber, burst transmission is used for an up-signal output from each subscriber so that the up-signal conflicts with up-signals output from the other subscribers. Various optical transmission systems have been proposed in order that an optical transmitter for up-signals supports the burst transmission, and a feed-forward type APC has been frequently used as one of the optical transmission systems.
A conventional optical transmitter is disclosed in Japanese Patent Publication No. 11-135871, for example. FIG. 2 is a block diagram showing the construction of the optical transmitter disclosed in the above publication. The temperature detected by temperature sensor 7 is converted to the value corresponding to the temperature at A/D-converter 8, and then input to memory 20. The memory 20 is stored with the value corresponding to pulse current Iac for pulse-driving LD (Laser Diode) 1 according to the value corresponding to the temperature and the value corresponding to bias current Idc for bias-driving LD1, therefore, the value corresponding to the pulse current Iac and the value corresponding to the bias current Idc according to the value corresponding to the surrounding temperature of the optical transmitter are output from the memory 20.
The value corresponding to the pulse current Iac is D/A-converted at D/A converter 6, and the output of the D/A converter 6 is input into current control circuit 4. The current control circuit 4 converts the value corresponding to the pulse current Iac into the pulse current Iac, and the pulse current Iac is input into driver 2. The driver 2 controls ON/OFF of LD1 emitting light according to input signal A (data input). At this time, the modulated current becomes the pulse current Iac. The value corresponding to the bias current Idc is D/A-converted at D/A converter 5, and the output of the D/A converter 5 is input into current control circuit 3. The current control circuit 3 converts the value corresponding to the bias current Idc into the bias current Idc, and the bias current Idc is supplied to the LD1.
As described above, the PON system burst-transmits the up-signals. In the burst transmission, data are randomly transmitted in a data section of a regulated time unit called as a burst cell. As the number of burst cells may be set to any value, a case where light emission occurs substantially continuously (FIG. 3A) is randomly intermingled with a case where light emission occurs sparsely (FIG. 3B). Even though the surrounding temperature of the optical transmitter is constant, the calorific value is varied in proportion to the amount of light emission from LD, as the density of burst cells is high or low. In the case where the density of the burst cells is high (FIG. 3A), the amount of current flowing in LD is large, and thus the self-calorific value of LD increases. In the case where the density of the burst cells is low (FIG. 3B), the time when the current flows in LD is short, and also the amount of the current is small, and thus the self-calorific value of LD decreases. That is, even though the surrounding temperature of the optical transmission is constant, the peak power of the light output is varied, as the density of burst cells is high or low. The density (high or low) of the burst cells is replaced with the magnitude (large or small) of the mark rate.
It is required to the optical transmitter for performing the burst transmission that the waveform instantaneously rises up from 1 bit of the burst head. However, in the conventional feed-forward type APC, only the surrounding temperature of the optical transmitter is detected, so that variation of the light output due to factors other than the surrounding temperature such as the magnitude of the mark rate or the like cannot be suppressed.
Therefore, an object of the present invention is to provide a circuit for driving a laser diode which having a feed-forward type APC circuit that can compensate variation of the peak power of the light output due to variation of a mark rate, and a method for driving a laser diode.
In order to attain the above object, the average value of an input signal A is detected by an average value detector, and the value corresponding to the mark rate thereof is input to a memory control circuit. The memory control circuit is connected to plural memories (memory A, memory B, memory C). Each of the memories stores data having different characteristics according to the temperature with respect to the value corresponding to pulse driving current Iac of a laser diode and the value corresponding to bias driving current Idc of the laser diode. The memory control circuit selects a memory to be used according to the mark rate detected, reads out the value corresponding to the pulse driving current Iac and the value corresponding to the bias current Idc according to the temperature from a temperature sensor, and determines the pulse driving current Iac and the bias current Idc. That is, by detecting the mark rate in the average value detector, the memory control circuit adjusts the pulse driving current Iac and the bias driving current Idc on the basis of the mark rate, and controlling the light output and the quenching rate of the laser diode to be constant.